Uranium.sup.238, in a series of radioactive reactions, decays eventually to lead.sup.206. Most of these reactions produce substances which undergo further radioactive decay. Radon.sup.222 represents one of these radioactive daughters of the decay products of uranium.sup.238. Specifically, radium.sup.226, also a member of the series, yields an alpha particle to give radon.sup.222.
Two important characteristics of radon.sup.222 necessitate its facile and accurate detection in the environment. First, all of the uranium.sup.238 that passes through the chain or reactions leading to lead.sup.206 must, at some time, yield radon.sup.222. By way of constrast, some of the reactions in the series may proceed through alternative paths, with only a small fraction of a parent substance proceeding through one of the paths. Thus, for example, the series of reactions also produces radon.sup.218. However, less than 0.001 percent of the decaying uranium.sup.238 and its daughter radionuclides provide radon.sup.218. Most of the material bypasses radon.sup.218 on its path to becoming lead.sup.206. However, substantially all of the nuclei produced by the decay of uranium.sup.238 must, eventually, take the form of radon.sup.222. Thus, the chain of radioactive decay reactions beginning with uranium.sup.238 assures a high rate of production of radon.sup.222.
Secondly, of the radionuclides produced in quantity by the decay of uranium.sup.238 and its daughters, only randon.sup.222 exists in the gaseous form; the others exist in the solid state. Since radon.sup.222 does not remain solid, it may depart from the place of its birth to contaminate the environment about it.
Not all radon.sup.222 produced from radium.sup.226, however, contaminates the environment. Frequently, it cannot leave a solid substrate surrounding the particular atom of radium.sup.226 which generated it. However, geological disturbances, such as the usual mining processes, can liberate the radioactive radon.sup.222. Techniques involving the use of copius amounts of water contacting with the material mined can dissolve appreciable amounts of radon.sup.222 and produce the possibility of contaminating a water supply.
Consequently, the detection of radon.sup.222 in the ambient atmosphere represents an important task. One method of accomplishing this involves the use of a scintillation cell which receives a sample undergoing testing for the presence of the radionuclide. Any radon.sup.222 present will, to some extent, radioactively decay to produce alpha particles. The daughter nuclides of radon.sup.222 can also decay to produce further alpha particles. All of these alpha particles then interact with luminescent material in the cell, causing it to scintillate and release light pulses. The light created travels through a transparent window to a detector which responds to it, such as a photomultiplier tube. A particularly advantageous example of such a scintillation cell appears in the patent application Ser. No. 805,629 of William M. Stevens, entitled "Strengthened Scintillation Cell", filed June 13, 1977.
The photomultiplier tube and the electronic equipment to which it attaches then coverts the produced pulses of light into an indication of the amount of radon.sup.222 present. Typically, this equipment includes calibrations and adjustments that allow it to correctly interpret the light pulses and provide an accurate indication as to the amount of radon present.
With the passage of time, however, the output indications of the instrument may change and report erroneously the amount of radon present. Consequently, it should receive occasional recalibrations to make certain that the output correlates with the actual amount of radon.sup.222 in the sample.
In particular, the instrument must respond properly to the entire amplitude spectrum of the light produced by the scintillation of the alpha particles. To verify the instrument's proper response, the detector should receive light at the wave lengths and intensities produced by radon's alpha particles. The ideal reference would, of course, utilize the alpha particles from a known concentration of radon.sup.222 striking the same luminescent material as in the sample cell. However, radon.sup.222 displays many characteristics that diminish its desirability as a standardizing reference source.
Initially, radon.sup.222 has the relatively short half life of 3.825 days. Accordingly, a reference cell containing radon.sup.222 would display substantially no activity even after the passage of as short a time as a few weeks. Consequently, a reference cell utilizing radon.sup.222 would require replenishment at frequent intervals.
M. Blau et al., in their U.S. Pat. No. 2,510,795, show a device which removes the radon produced from radium and freezes it in a separate container. However, they then retain the radon until it decays into its radioactive daughters, in which form it may then find use for such devices as vacuum tubes. Gerhardt, in his U.S. Pat. No. 3,774,036, shows an apparatus for generating and maintaining a supply of a short-lived daughter radionuclide from a parent having a longer half life. A substantially different apparatus, but with a similar purpose, appears in U.S. Pat. No. 3,912,935 O. A. Harris.
O. Hahn, in U.S. Pat. No. 1,655,184, provides precipitate mixtures including radium salts. The produced precipitates have a fine state of subdivision and allow the radon.sup.222 decay products to freely escape from the solid lattice. The radon may then find subsequent use for medical purposes.
Utilizing a generator for radon, such as the above, would impose a tremendous burden upon the investigator employing radon-detection equipment. He would frequently have to stop his activities to generate the radon before he could adjust his intruments.
Moreover, providing the radon does not represent a simple task. This results from the fact, discussed above, that radon exists in the gaseous form. Thus, the investigator would have to employ gas-tight equipment to generate the radon. Not only would it take appreciable amounts of his time, but he might have to return to a laboratory which could provide the proper apparatus. Furthermore, he would have to do this frequently since the produced radon, as discussed above, has a half life of less than four days.
B. A. Staples, in his U.S. Pat. No. 3,859,179, provides a method of preparing a calibration source for film badges sensitive to beta particles. Specifically, he electroplates a stainless steel disc with nickel and a carrier including radioactive rhuthenium.sup.106. This radionuclide decays with a half life of one year to rhodium.sup.106. The latter isotope has a half life of 30 seconds and emits beta particles with an energy of 3.55 MeV. These beta particles calibrate the film badges to indicate the amount of radiation associated with the particular degree of darkening of the film.
However, neither of the isotopes involved in the calibration source of Staples provides alpha particles. Furthermore, Staples does not have to contend with either of the isotopes existing in the gaseous state. Consequently, he does not help adjust an alpha-scintillation counter.